This application claims priority, under 35 U.S.C. 119, of earlier-filed Italian Application TO2001A000133, filed Feb. 15, 2001.
The present invention relates to a rocker arm for valve trains of internal-combustion engines, and more particularly, to rocker arms for use in valve gear trains of the xe2x80x9cend-pivotxe2x80x9d rocker arm type.
For a better understanding of the state of the art regarding the subject in question and the problems relating thereto, firstly a rocker arm of known design will be described, with reference to FIGS. 6, 7A and 7B of the accompanying drawings.
FIG. 6 is a view, partially sectioned longitudinally, of a valve train, generally designated 1, which is able to cause the alternating rectilinear movement of an engine poppet valve (only a stem 20 of the valve being shown in FIG. 6) in accordance with a predetermined opening sequence. The valve train comprises a rocker arm 2, a hydraulic tappet 3 and a cam-type actuating member 4.
The mutual arrangement of the above-described components may vary depending on the type of engine and the type of distribution chosen. In particular the present invention relates to rocker arms of the type comprising end portions 5 and 6 able to engage the tappet 3 and the valve, respectively, and an intermediate portion 7 intended to receive a roller 8 co-operating with the cam-type actuating member 4. An example of an embodiment of a rocker arm of this type is illustrated in detail in FIGS. 7A and 7B which show longitudinally sectioned and cross sectional views thereof, respectively.
The operating principle of a valve train of the above-mentioned type is well-known to a person skilled in the art: the rotational movement of a cam shaft (which is not shown, but which rotates the cam-type actuating member 4) is converted into the alternating rectilinear movement of the valve. Such rectilinear movement is the result of the interaction between the cam member 4, having a base circle portion 9 and an eccentric profile (lift portion) 10, and the roller 8 of the rocker arm, said interaction acting so as to cause oscillation of the rocker arm in its own longitudinal plane of symmetry (coinciding with the plane of the sheet showing FIG. 6), about a fulcrum point located in the zone of contact between the rocker arm 2 and the tappet 3.
At present this type of rocker arm is advantageously produced by means of the operations of:
(a) shearing of a shaped element 30 (see FIG. 8), from a sheet of steel with a low carbon content, the element 30 having a form symmetrical with respect to a longitudinal axis 35 and being provided with an opening 36, in a substantially intermediate position, and with two holes 21 and 22 situated laterally with respect to the opening 36;
(b) pressing the above-mentioned shaped element 30 in order to perform bending upwards (or downwards) of lateral portions 31 and 32, along bending lines 33 and 34, respectively, so as to provide the part with a substantially U-shaped cross section (see FIG. 7B), having a horizontal plate portion 11 which connects two vertical side walls 12 and 13;
(c) forming the horizontal plate portion 11, at the end 5 of the rocker arm, so as to produce a partly spherical portion 14 having a concave surface of revolution 15 with an essentially ogive-shaped section able to engage with an essentially hemispherical convex outer surface 16 at the top of the hydraulic tappet 3;
(d) forming the above-mentioned horizontal plate portion 11, at the end 6 of the rocker arm, so as to produce a shoe element 17 having a surface 18 with its concavity directed downwards and an arched cross section (in the plane of oscillation of the rocker arm), able to interact with the top (tip) 19 of the stem 20 of the engine poppet valve; and
(e) inserting and locking a cylindrical pin 23 in the two seats defined by the above-mentioned holes 21 and 22, the roller 8 being rotatably mounted on the pin 23 by means of rolling elements 24 so as to project partially from the opening 36 in order to engage with the cam member 4.
The shape and dimensions of the rocker arm 2 are dictated by the design requirements of the engine manufacturer and must therefore be able to satisfy precise geometrical constraints associated with predetermined positions, in the engine cylinder head, of the other valve train elements with which the rocker arm 2 must co-operate. The geometrical constraints determine the arrangement, in the plane of longitudinal symmetry of the rocker arm (coinciding with its plane of oscillation), of three significant points A, B and C, indicated in FIG. 6, as follows:
A is a center of the theoretical circumference (or hemisphere) of contact between the engaging surfaces 15 and 16 of the rocker arm 3 and the tappet 4, respectively;
B is a center of the pin 23 of the roller 8; and
C is a theoretical point of contact between the contact surface 18 of the rocker arm 2 and the contact surface on the valve tip 19 of the valve stem 20.
If the design requirements of the engine cylinder head result in the positioning of the above-mentioned point B at a sufficiently large lateral distance from the straight line passing through the other two points A and C at the opposite end zones 5 and 6 of the rocker arm 2, respectively, the rocker arm may be manufactured by means of simple shearing and bending operations, with low production costs.
In order to clarify this point, it should be noted, with reference to FIG. 8, how the central opening 36 of the semi-finished product 30 has an elongated shape in the longitudinal direction, with an intermediate section 37 having a transverse dimension, or width, which is smaller than that of two longitudinal end sections 38 and 39 and how the two holes 21 and 22 are positioned opposite the above-mentioned intermediate section 37. The width of the intermediate section 37 of the opening 36 cannot be less than a certain minimum value imposed by the technological constraints associated with the feasibility of the shearing operation. Consequently, the width of internal flanges 41 and 42 located between the intermediate section 37 and the holes 21 and 22, respectively, has an upper limit value, once the dimensions of the above-mentioned holes and their distance from the axis 35 have been fixed.
In the design situation where the center B of the pin 23 is located underneath the straight line joining the end points A and C, the operation of bending of the lateral portions 31 and 32 of the semi-finished product 30 into a xe2x80x9cUxe2x80x9d is performed downwards, along the bending lines 33 and 34. These bending lines, viewed in the longitudinal plane of symmetry of the rocker arm, are substantially parallel to the straight line passing through the points A and C. Observing, in FIG. 8, the geometry of the shaped element 30, it can be easily understood that, if the distance of the point B from the straight line passing through the points A and C is fairly large, then the maximum width of the flanges 41 and 42 is sufficient to perform the function of laterally containing the rolling elements 24 (usually rollers) of the roller 8.
When, on the other hand, the center of the pin 23 (point B) must be located above the straight line joining the end points A and C, the above-mentioned operation of bending into a xe2x80x9cUxe2x80x9d shape is performed upwards, again along the lines 33 and 34. The two internal flanges 41 and 42 are thus positioned, at the end of bending, underneath the holes 21 and 22 of the pin and therefore must no longer perform the function of laterally containing the rolling elements of the roller, but must ensure the necessary flexural stiffness of the rocker-arm body. If, therefore, the distance of the point B from the straight line A-C, i.e. the distance of the centers of the holes 21 and 22 from the bending lines 33 and 34, respectively, is too small, the maximum width of the flanges 41 and 42 may not be sufficient to provide the rocker arm with the required rigidity.
The problem of how to produce a rocker arm by means of pressing therefore arises, in particular, when the design constraints require a substantially aligned position of the three above-mentioned points A, B and C, i.e. essentially the top of the tappet, center of the roller and top of the valve stem.
Known prior art solutions envisage in this case the bending, for example downwards, of the lateral portions 31 and 32 of a shaped element 30 similar to that of FIG. 8, along bending lines which, viewed in the plane of longitudinal symmetry of the rocker arm, no longer substantially coincide with the straight line passing through the end points A and C, but are inclined upwards through an angle such as to ensure a width of the flanges 41 and 42 sufficient for performing the function of containing the rolling elements of the roller. It is therefore necessary to perform a further operation involving plastic deformation in order to displace the end portion 5 of the horizontal plate portion 11 of the rocker arm, which at the end of this first operation is still located aligned with the bending line, downwards as far as the level of the point C. This process is costly since it requires the use of presses capable of generating very high forces.
Accordingly, it is an object of the present invention to provide a rocker arm which, even in the case of substantially aligned positioning of the above-mentioned three points A, B and C, may be produced through simple bending into a xe2x80x9cUxe2x80x9d shape by means of pressing without the need for further plastic deformation operations which produce a relative displacement of the end portions 5, 6 and the intermediate portion 7 of the rocker arm.
These and other objects and advantages, which will emerge more clearly from the following description, are achieved by providing an improved rocker arm of the type constructed by means of deformation of a shaped element made of metallic material, comprising surfaces for engagement with a tappet and with a stem of a valve, respectively, and a portion for mounting of a rotatable roller, able to co-operate with a cam-type actuating member.
The improved rocker arm is characterized by the fact that the surface of engagement with the hydraulic tappet is formed in an insert fixed to the rocker arm. In accordance with a more limited aspect of the invention, the insert is formed, dimensioned, and positioned so that the surface for engagement with the tappet is essentially aligned with the roller and with the surface for engagement with the poppet valve.